What is a vitamin?

Answer 1

A vitamin is an organic compound required as a vital nutrient in limited amounts. Your body can't make vitamins in sufficient quantities, so you must obtain them from your diet or from supplements.

Each vitamin comprises one or more vitamer compounds grouped under a letter of the alphabet, such as "vitamin A", which includes the compounds retinal, retinol, and about 50 carotenoids. Your body can convert vitamers to the active form of the vitamin, and sometimes to other vitamers, as well.

Vitamins have many functions. Some have hormone-like functions as regulators of mineral metabolism (e.g., vitamin D), or regulators of cell and tissue growth and differentiation (e.g., some forms of vitamin A). Others function as antioxidants (e.g., vitamin E and vitamin C). The largest number of vitamins (e.g., B complex vitamins) help enzymes in their work in metabolism.

The value of eating a certain food to maintain health was recognized long before vitamins were identified, and the discovery of vitamins was a slow and fairly recent process. Vitamin deficiencies can cause diseases such as goiter, scurvy, osteoporosis, immune deficiencies, metabolic disorders, cancer, premature aging, and eating disorders, among many others. Excess of some vitamins, especially fat-soluble vitamins such as vitamin A, can be toxic.

Vitamins are classified as either water-soluble or fat-soluble. In humans there are 13 vitamins: 4 fat-soluble (A, D, E, and K) and 9 water-soluble (8 B vitamins and vitamin C). Your body can use most vitamins for multiple functions.

Dr. T. Colin Campbell argues that vitamin A as retinol isn't a true vitamin, because the human body can manufacture retinoids out of the true essential vitamin, the carotenoids. He also asserts that vitamin D isn't a true vitamin, because the human body can synthesize it from the action of sunshine on the skin.